Many kinds of cells can be grown in culture, provided that suitable nutrients and other conditions for growth are supplied. Thus, since 1907 when Harrison noticed that nerve tissue explanted from frog embryos into dishes under clotted frog lymph developed axonal processes, scientists have made copious use of cultured tissues and cells from a variety of sources. Such cultures have been used to study genetic, physiological, and other phenomena, as well as to manufacture certain macromolecules using various fermentation techniques known in the art. In studies of mammalian cell biology, cell cultures derived from lymph nodes, muscle, connective tissue, kidney, dermis and other tissue sources have been used. Generally speaking, the tissue sources that have been most susceptible to the preparation of cell cultures for studies are derivatives of the ancestor mesodermal cells of early development. Tissues that are the progeny of the ancestor endodermal and ectodermal cells have only in recent years become amenable to cell culture, of a limited sort only. The cell types derived from the endoderm and ectoderm of early development include epidermis, hair, nails, brain, nervous system, inner lining of the digestive tract, various glands, and others. Essentially, long-term cultures of normal differentiated glandular and epithelial cells, particularly those from humans, are still not available.
In the instance of the mammalian pancreas, until the present invention, no scientist has had the opportunity of studying, and no physician has had the prospect of using for treatment, a cell culture of pancreatic endocrine cells that exhibited sustained cell division and the glandular properties typical of the pancreas.
Similar to neurons, the endocrine cells of the mammalian pancreas have been considered to be post-mitotic, i.e., terminal, essentially non-dividing cells. Recent work has shown that the cells of the mammalian pancreas (including those of humans) are capable of survival in culture, but are not capable of sustained cell division. Hence, a primary culture of the tissue cells can succeed, but due to a lack of sufficient cell divisions of the cultured cells, passaging of the primary culture to form serial cultures has not been possible. Although occasional cells in a metaphase stage, uptake of tritiated thymidine, and other evidence of cell division have been seen in these cultures (Clark et al., Endocrinology, 126:1895 (1990); Breljie et al., Endocrinology, 128:45 (1991)), the overall rate of cell division has been considered to be below the replacement rate (that is, more, or as many, cells die as are produced). Therefore, pancreatic endocrine cell cultures prior to the present invention were not expanded.
The inability to study pancreatic endocrine cells in culture has impeded the ability of medical science to progress in the area of pancreatic disorders. Such disorders include diabetes mellitus, a disease that impairs or destroys the ability of the beta cells of the islets of Langerhans (structures within the pancreas) to produce sufficient quantities of the hormone insulin, a hormone that serves to prevent accumulation of sugar in the bloodstream. Type I diabetes mellitus (insulin dependent, or juvenile-onset diabetes) typically requires full hormone replacement therapy. In a second (and more common) form of the disease, type II diabetes (sometimes referred to as late onset, or senile diabetes), treatment often does not require insulin injections because a patient suffering with Type II diabetes may be able to control his/her blood sugar levels by carefully controlling food intake. However, as many as 30% of these patients also have reduced beta cell function and therefore are candidates for hormone replacement therapy as well. Diabetes is not confined to humans, but has been noted in other mammals as well, such as dogs and horses.
The etiology of the diabetic disease condition is not fully understood. However, it has been noted that autoimmunity antibodies (antibodies that "mistakenly" attack bodily structures) and/or certain T lymphocytes may have an involvement long before clinical symptoms of diabetes emerge. Evidence in this direction relies, in part, on successful treatment of recently diagnosed diabetic patients with cyclosporin, an immunosuppressive drug. Such treatment has been shown to prevent or cause remission of insulin-dependent diabetes mellitus in mice (Mori et al., Diabetologia 29:244-247 (1986)), rats (Jaworski et al., Diabetes Res. 3:1-6 (1986)), and humans (Feutren et al., Lancet, 11:119-123 (1986)). A clinical test to detect the presence of these humoral and cellular immunoreactions would allow the screening of individuals in a pre-diabetic state, which individuals could then be prophylactically treated with immunosuppressive drugs.
Current treatment of individuals with clinical manifestation of diabetes attempts to emulate the role of the pancreatic beta cells in a non-diabetic individual. Individuals with normal beta cell function have tight regulation of the amount of insulin secreted into their bloodstream. This regulation is due to a feed-back mechanism that resides in the beta cells that ordinarily prevents surges of blood sugar outside of the normal limits. Unless blood sugar is controlled properly, dangerous, even fatal, levels can result. Hence, treatment of a diabetic individual involves the use of injected bovine, porcine, or cloned human insulin on a daily basis.
Injected insulin and diet regulation permit survival and in many cases a good quality of life for years after onset of the disease. However, there is often a gradual decline in the health of diabetics that has been attributed to damage to the vascular system due to the inevitable surges (both high and low) in the concentration of glucose in the blood of diabetic patients. In short, diabetics treated with injected insulin cannot adjust their intake of carbohydrates and injection of insulin with sufficient precision of quantity and timing to prevent temporary surges of glucose outside of normal limits. These surges are believed to result in various vascular disorders that impair normal sight, kidney, and even ambulatory functions.
Both of these disease states, i.e., type I and type II diabetes, involving millions of people in the United States alone, preferably should be treated in a more regulated fashion. Successful transplants of whole isolated islets, for example, have been made in animals and in humans. However, long term resolution of diabetic symptoms has not yet been achieved by this method because of a lack of persistent functioning of the grafted islets in situ. See Robertson, New England J. Med., 327:1861-1863 (1992).
For the grafts accomplished thus far in humans, one or two donated pancreases per patient treated was required. Unfortunately only some 6000 donated human pancreases become available in the United States in a year, and many of these are needed for whole pancreas organ transplants (used when the pancreas has been removed, usually during cancer surgery). Therefore, of the millions of diabetic individuals who could benefit from such grafts, only a relative handful of them may be treated given the current state of technology. If the supply of islet cells (including but not necessarily limited to beta cells) could be augmented by culturing the donated islets in cell culture, expanded populations would provide sufficient material to allow a new treatment for insulin-dependent diabetes.
In a similar fashion, the follicle cells of the human thyroid gland are highly specialized to respond to ambient levels of thyroid stimulating hormone, TSH, and to synthesize thyroglobulin, a very large complex protein that requires iodination for its activity. In response to TSH levels, thyroglobulin is secreted as tetra-iodo and tri-iodo thyronine (T.sub.3), which are known collectively as the thyroid hormone, thyroxine. The thyroid cells of rats have been successfully cultured in media that allows the specialized functioning as well as the hormone dependence of these cells to be retained (Ambesi et al., Proc. Natl. Acad. Sci. USA, 77:3455-3459 (1980)); however, analogous cell cultures of human thyroid cells have not been successfully maintained. These rat cell cultures, called FRTL and FRTL-5, and their clonal variants have become the basis for clinical tests that seek to identify thyroid stimulating substances in the serum of patients with suspected thyroid disease. The FRTL/FRTL5 cell cultures originated from normal adult rat thyroid glands. These cell strains respond to thyrotropin (TSH) by releasing thyroglobulin (Tg), producing cyclic AMP (cAMP), trapping iodide, and growing. The TSH-dependent growth in FRTL and FRTL5 cells suggested a key role of the hormone as a mitogenic factor for thyroid cells; however, not all reports have confirmed this observation (see Westermark et al., Proc. Natl. Acad. Sci. USA, 76:2022-2026 (1979); Valente et al., Endocrinology, 112:71-79 (1983)). As to the role of cAMP, as a second messenger, it appears that components besides the modulation of cAMP production may be involved in TSH stimulatory effects (see, for example, Lombardi et al., Endocrinology, 123:1544-1552 (1988)). Whereas in genetically engineered FRTL5 cells a pseudo-physiological rise of intracellular cAMP level is enough to stimulate cells proliferation (Hen et al., Proc. Natl. Acad. Sci. USA, 86:4785-4788 (1989)), normal thyroid cells cultured from other sources may not display the same behavior.
Other second messengers, besides cAMP, have been hypothesized to have a role in the regulation and action of thyroid cells; however, no clear empirical data support any such hypotheses (see, for example, Raspe et al., Mol. Cell. Endocrin., 81:175-183 (1991)). An important role may also be played by autocrine (Takahoshi et al., Endocrinology, 126:7-36-7-45 (1990)) or indirect paracrine influences (Goodman and Rene, Endocrinology, 121:2131-2140 (1987)). Little can be recited definitively because the above-cited studies dealt with thyroid cells from different animal species or from human pathological samples so that discrepancies may be due to differences between species, to the various pathological conditions, or to adaptation of the cells to the various culture conditions used. The few studies on reportedly normal, non-transformed donor tissues have been to primary cultures, with very little evidence of in vitro cell proliferation (see, for example, Raspe et al., supra).
Thyroid pathologies, such as goiter, Grave's disease, Hashimoto's disease, adenomas, and carcinomas, involve impairment of thyroid function and, typically, excision of the thyroid itself. While the etiology of thyroid pathologies are not well understood, treatment post-excision focuses on a hormone-replacement-based therapy. If normal thyroid cells could be produced in culture in sufficient quantities, such expanded populations would provide sufficient material to allow a post-excision new treatment for these thyroid diseases.
When the thyroid gland is damaged or removed, often the parathyroid glands are also damaged or removed. While the function of the thyroid gland is rather successfully replaced by taking thyroid hormone by mouth, the parathyroid function is not easily replaced. The principal hormone product of the parathyroid gland is a protein hormone called parathormone that is not effective if taken by mouth. Parathormone interacts with vitamin D and regulates mineral metabolism, particularly calcium.
A similar situation exists with respect to the parotid glands. These glands are located in the angle of the jaw and are responsible for producing much of the saliva that lubricates the oral cavity. In particular, three major salivary proteins are secreted by the parotid gland; namely, lumicarmine, amylase, and gustin. The absence of the parotid secretions can result in xerostomia, or dry mouth, a common, clinically disturbing but not life-threatening disorder. Xerostomia affects all patients following X-irradiation of the oral cavity for treatment of oral cancers and many patients with Sjogren's syndrome. This disorder exacerbates symptoms of stomatitis, gingivitis, periodontitis, taste loss and tooth loss. Treatment of this symptom has been largely unsuccessful, consisting mainly of supplying oral moisturizers. If normal parotid cells could be produced in culture in sufficient quantities, such expanded populations would provide sufficient material to allow a new treatment for the xerostomic disorder.
Other cell types have been similarly refractory in being cultured long-term by conventional methods, particularly those of ectodermal or endodermal embryonic derivation. Among these other cell types are cells of the olfactory neuroblasts, prostate gland, lachrymal gland, cartilage, inner ear, liver, parathyroid gland, oral mucosa, sweat glands, hair follicles, adrenal cortex, urethra, bladder, many human tumors, and others. Additionally, primary human tumor cells have not been susceptible to propagation in culture, including those tumor cells of the thyroid, lung, cervix, epithelium (carcinoma), and pituitary and thyroid adenoma.
Some cell types, such as amniocytes and venous and arterial endothelium, have been cultured in vitro; however, the growth rates or the faithful retention of differentiated functions have not proven particularly efficacious. Growth rates of amniocytes in conventional media are such that the time required to grow the cells for purposes of diagnosis of some genetic disorders can result in providing information at a time point in the development of a fetus, for example, when the information can be acted upon only with the most dire of impact on the patient, or, perhaps, cannot be acted upon at all. Such growth rates have an economic impact, of course, with respect to the culturing of any of the aforementioned cells. To the extent the cultured cells themselves are products for surgical procedures, for example, skin cells applicable to burn victims, or for production of pharmaceuticals, the existence of techniques to cause cell culturing rates to increase results in a more plentiful and less costly supply of those cells.
The present invention attempts to meet many of these culturing needs. In particular, the present invention provides a novel culturing method and medium which are capable of producing an expanded culture of a wide variety of cells which have previously not been so cultured. Such cells include pancreatic islet cells, thyroid cells, parathyroid cells, parotid cells, tumor cells, and the other cell types discussed above. The present invention further seeks to provide certain aggregates of cells, such as pancreatic, thyroid, parathyroid and parotid cells, that have tissue-like qualities (referred to herein as "pseudotissues"), as well as the use of such pseudotissues for the treatment of various disorders, e.g., blood sugar concentration disorders, thyroid deficiencies, parathormone deficiencies and/or mineral dyscrasia, and xerostomia in mammals. The present invention also seeks to provide techniques for the use of the cultured cells for cytotoxicity assays of exogenous materials and to assess disease states of patients.
These and other features and advantages of the invention will be more readily apparent upon reading the following description of preferred exemplified embodiments of the invention and upon reference to the accompanying drawings, all of which are given by way of illustration only, and are not limitative of the present invention.